Shadow Wraith

I was just curious about something, and I was hoping to get some feedback on something.

In high school biology (I took the class in my junior year, about 3 1/2 years ago), the subject of viruses caught my attention a great deal. It seems to me like they may be the missing links between nonliving and living things. They aren't quite living, but they're not quite nonliving either. It started to get me wondering about the origins of life. Could the chemical evolution of prebiological Earth result in the eventual formation of the complex proteins that we would identify as viruses (or something similar)? Could such things eventually become more complex so as to eventually become what we consider to be the simplest early life forms, such as procaryotes and eucaryotes(sp?) among other bacteria/protist-like things? Are living things nothing more than essentially a form of self-replicating matter? I though such an idea sounds like it should be worth investigating as a possible idea for life's origins. It may or may not be good, though. I was better in the physical sciences than the life sciences anyway .

CodeMason

It's generally agreed that viruses are an evolutionary oddity that branched off as a byproduct of very early life, but not a "bridge between life and nonlife" for the simple reason that virii require a host. Therefore the "hosts" themselves must have come first out of necessity.

Quetzal

You're both right.

Modern viruses are pure parasites that are incapable of surviving long outside their particular hosts or vectors (except for the ones capable of forming viral spores). However, something very like a virus - a simple strand of RNA surrounded by a lipid sheath - is one of the leading contenders for the first "living" organism, and may have evolved into the first bacteria by gene-swapping and symbiosis.

Another interesting tidbit, viruses are also responsible for many of the oddball organelles of modern bacteria - for example, cillia and flagella. They form the building blocks of many modern bacteria, and the foundation of the Serial Endosymbiosis Theory.

If they weren't so deadly to us, viruses could be considered pretty cool.

CodeMason

I thought it was the endosymbiosis of bacteria that is responsible for the organelles.

Blinn

Yeah, mitochondria and chloroplasts anyway.

Coragyps

Bacteria as endosymbionts are what Margulis proposed as the ancestors of organelles in eukaryotes, IIRC, and a lot of evidence points that way. Morpho, do you have any references for viruses as bacterial organelle precursors? That just sounds absolutely too cool - like the old bit of doggerel about "fleas have lesser fleas to bite 'em ... and so ad infinitum."
(The memory is the second thing to go....)

Quetzal

Post in haste, regret in leisure...

codemason and zetek are correct: SET uses symbiosis of simple bacterial for formation of both mitochondria/plastids and motility. Worse still, I knew better. Sometimes you have brain burps...

To repost something I posted a while back: a modern example of SET in action is Mixotricha paradoxa. This bacteria looks like a single-celled swimming ciliate. With the electron microscope, however, it is seen to consist of five distinct kinds of creatures. Externally, it is most obviously the kind of one-celled organism that is classified as a protist. But inside each nucleated cell, where one would expect to find mitochondria, are many spherical bacteria. On the surface, where cilia should be, are some 250,000 hairlike Treponema spirochetes (resembling the type that causes syphilis), as well as a contingent of large rod bacteria that is also 250,000 strong. In addition, 200 spirochetes of a larger type, Canaleparolina darwiniensis are found inside the average M. paradoxa

An excellent (relatively non-technical) treatment of SET can be found <a href="http://members.tripod.co.uk/taduolus/Symb.htm" target="_blank">here</a>.

Again, apologies for being an idjut.

Grumpy

Are viruses a good counter-example of Irreducible Complexity? If one believed that viruses were precursors to all other life, one would face the unavoidable paradox that they could not have existed prior to the more complex descedant. It makes sense, however, to suppose that they developed later, with the help of an infrastructure of more complex organisms.

CodeMason

The best hypothetical counter-example to IC I can think of from origins of life research is the RNA World. The DNA-RNA-replicase system is irreducibly complex, however origins researchers have shown how this system could have very easily evolved step-by-step, starting with RNA.

tronvillain

Morpho:
I know I bring this up every time, but I'm still a fan of the idea that the nucleic acid strands started as parasites, then became symbiotes, then became central to replication.

[ January 10, 2002: Message edited by: tronvillain ]</p>